V-PULL-APART MICROSTRUCTURES - A NEW SHEAR-SENSE INDICATOR

Granitoids deformed in a low-grade shear zone show conspicuous develop ment of asymmetric pull-apart microstructures in feldspars, characteri zed by 'V'-shaped gaps between the separated fragments ('V'-pull-apart s). They occur beside active shear planes and result from rigid-body r otation of one fragment relative to the other. The shear sense in the adjacent flow surface can be easily determined through the asymmetry n ormally shown by 'V'-pull-aparts, where the fracture wall at a lower a ngle to the shear plane indicates which fragment has rotated synthetic ally to the adjacent shearing. Another diagnostic feature is the folia tion deflection that occurs adjacent to the rotated fragment. The 'V'- shaped gaps are infilled by recrystallized quartz-mica aggregates, sho wing quartz c-axis patterns indicative of strong participation of crys tal-plastic processes in these domains. Evidence of crack-seal by prec ipitation (c-axes parallel to the direction of separation) have also b een found. The rigid-body rotation indicated by these microstructures, worked as a mechanism of removal of protruding porphyroclast edges, e nabling the smoothing of the shear planes. Rigid-body rotation should operate in circumstances where other mechanisms such as dissolution or crystal-plasticity did not succeed in eliminating undulations along t he shear planes.